The present invention relates to a circuit for the protection of plasma-arc welding/cutting equipment operating with transferred or non-transferred arc. The prior art embraces plasma-arc welding systems with torches that comprise a cylindrical electrode, rigidly attached to the body of the torch itself and connected by way of a conductor to the negative terminal of the power source. Such torches further comprise an anode in the form of a cap, mounted in fixed position with respect to the electrode and covering the tip; the cap is insulated from the tip and connected by way of a second conductor to the positive terminal of the power source.
Cap and electrode are positioned in such a way that a thin gap is left between them for admission of the pressurized stream of plasma gas, which emerges ultimately by way of a pin hole in the cap; thus, the cap is known generally as the `nozzle`. In a first conventional method of using torches of this type, an arc is struck with the assistance of an ultra-high auxiliary voltage, generated in most instances by complicated high frequency circuits with which the torch is connected by long, heavily insulated conductors.
More exactly, a spark is ignited between the nozzle and the electrode, these being fixed in relation to one another, from which the resulting arc remains struck until cut off by operating the torch control button or the main power switch.
In the majority of instances, this non-transferred arc type of operation, so called by reason of the fact that the electric arc remains confined to the torch, is converted to transferred arc operation by connecting the work for welding or cutting to the same positive terminal as that to which the return conductor from the nozzle is connected, and wiring a resistance between the terminal and the return. As the torch is offered to the work, the arc will encounter less resistance from the work than from the return terminal, and accordingly is transferred to the work. The same effect can also be obtained by wiring a switch in place of the resistance, of which the contacts will be broken as welding or cutting commences.
In a second system of cutting or welding by the transferred arc method, the arc is struck between the electrode and the work, the electrode again incorporated fixedly into the torch and connected to the negative terminal of the power source, and the work connected to the positive.
This type of torch has a movable nozzle, and the arc is struck simply by bringing about a momentary contact of the nozzle with the electrode on the one hand and the work (which is grounded) on the other. The effect of contact and immediate release is to bring about an instantaneous short circuit of the welding current as the electrode, nozzle and work are bridged, whereupon an arc is struck across the gap which opens up as the nozzle is distanced from the electrode, and transferred immediately from the electrode to the work.
Needless to say, if an arc is to be struck and held between the two elements of the torch, a singularly high potential difference must exist between the two.
Conventionally, plasma-arc torches of the type in question comprise an electrode that consists of a copper holder, and a central insert from which the arc is effectively struck; accordingly, the insert is fashioned in material possessing particularly good resistance to heat, for example zirconium and hafnium, and will thus be capable of withstanding high temperatures without sublimating.
As a general rule, the p.d. needed between cathode and anode to sustain an arc, or arc voltage, has been found by experiment to be equivalent to the sum of three distinct voltage drops--i.e. through the cathode, through the anode and through the arc, the drop through the arc being proportionate to the length of the arc itself.
The drop in cathode voltage is by far the greatest, and, being concentrated in the area immediately surrounding the electrode across a singularly short space (of the order of a few .mu.m), characterized by the presence of a strong electric field, hence by high losses; temperatures thus rise to particularly high values in the area in question, and it is for this reason that the electrode insert is fashioned from a material with a high sublimation point.
Accordingly, during cutting or welding operations, the electrode becomes subject not only to stresses of an electrical nature, but also to marked thermal stresses, hence to wear.
More exactly, as the insert is gradually consumed, the contour of the encompassing copper holder is laid bare, with the result that the copper itself becomes involved, unwarrantably, in generation of the arc.
Being a metal of relatively low melting point, in comparison to that used for the insert and found thus undesirably to be conductive and fuelling the arc, the holder begins to melt and shed molten material; the result is that a crater is formed in the electrode, upsetting its geometry, and one has a steady deterioration comparable in its effect to that of an avalanche.
Deformation of the electrode continues, and the characteristics of the plasma envelope are modified to the extent that the encompassed arc becomes destructive.
In particular, the voltage generated between the electrode and the nozzle of a plasma-arc cutting or welding torch, both on striking the arc and holding it thereafter during operation, depends solely upon the distance by which the two parts are separated; thus, a variation in this distance, or gap, will produce a corresponding rise or fall in voltage. In practice, factors which tend to alter the gap between electrode and nozzle include the effects of impurities in the air fed into the plasma stream, of the particles of molten metal that separate from the electrode and carry into the plasma envelope, and of wear on the electrode.
More exactly, continual melting and dispersion of the electrode copper can lead ultimately to the formation of a bridge between electrode and nozzle, the effect of which is to short-circuit the two components and produce a collapse in the voltage level, which gravitates swiftly towards zero. In a situation such as this, both the torch and the equipment power source can become severely damaged.
Accordingly, it is most important to be able to monitor the state of wear of the insert, hence of the entire electrode, to the end of preventing the deterioration described above and avoiding its attendant drawbacks.
The object of the present invention is to provide a protective circuit applicable to plasma-arc torches of the type in question and capable of overcoming the difficulties aforementioned.